I'll be the first to point out if any compelling evidence comes out to suggest it really was a complete accident, as opposed to an accident that had a little help. In the meantime there are some interesting articles going around in the French press, citing the ex-chief architect of the Notre Dame.
Notre-Dame : après l'incendie, "rien n'est fini" selon l'ex-architecte en chef
“I don’t understand this at all. The fire’s spread is extremely curious. With really old oak like that, it’s super hard, so you’d need a lot of smaller wood to first get the fire hot enough for the oak beams to burn.
In 2010 we replaced all the electrical wiring, so there’s no way this was sparked by a short-circuit. We put new wiring in place according to modern standards. And we went even further; we installed state-of-the-art fire protection and detection systems in the cathedral.
At all times, there are always two men on standby in the Cathedral, day and night, to go investigate anything the moment an alarm goes off, then if necessary to call the fire emergency services.
I am really stunned that this has happened. It’s incomprehensible. In 40 years of experience, I have never heard of a fire like this.
It would take a really combustible load at the start of such a fire for it to rapidly develop into the disaster we saw. Also, oak is a particularly fire-resistant wood.
The fire detection/protection system installed in the cathedral was of the highest level. It was a very expensive undertaking, but it enabled us to quickly ascertain whether a fire had started somewhere. Additionally, we had many old wooden doors replaced by fire doors.”
